Facebook is one of the biggest PHP users on the planet and they seem to think they can do more on their own, then the php community can do. In 2010, Facebook developers built HipHop as a newer/faster PHP runtime.
Now Facebook is going after security with the new XHP extension. The basic idea behind XHP is to make front-end code easier to understand and to help mitigate against Cross Site Scripting (XSS) attacks. In XHP, XML can be used inside of PHP.